The transfer of heat, whether by conduction,
convection or radiation, depends on temperature difference
- heat flows from hot to cold at a rate which depends on
the temperature difference between them. Thus, a cable installed
near the roof of a boiler house where the surrounding (ambient)
temperature is very high will not dissipate heat so readily
as one clipped to the wall of a cold wine cellar.

[Appendix 4] includes two tables giving
correction factors to take account of the ability of a cable
to shed heat due to the ambient temperature. The Regulations
use the symbol Ca to represent this correction factor. The
tables assume that the ambient temperature is 30°C and give
a factor by which current rating is multiplied for other
ambient temperatures.

For example, if a cable has a rating of
24 A and an ambient temperature correction factor of 0.77,
the new current rating becomes 24 x 0.77 or 18.5 A. Different
values are given depending on whether the circuit in question
is protected by a semi-enclosed (rewirable) fuse or some
other method of protection. The most useful of the correction
factors are given in {Table 4.3}.

In {Table 4.3}, '70°C m.i.' gives data
for mineral insulated cables with sheaths covered in p.v.c.
or LSF or open to touch, and '105°C m.i.' for mineral insulated
cables with bare sheaths which cannot be touched and are
not in contact with combustible material. The cable which
is p.v.c. sheathed or can be touched must run cooler than
if it is bare and not in contact with combustible material,
and so has lower correction factors.

Mineral
insulated cables must have insulating sleeves in terminations
with the same temperature rating as the seals used.

Where
a cable is subjected to sunlight, it will not be able to
lose heat so easily as one which is shaded. This is taken
into account by adding 200°C to the ambient temperature
for a cable which is unshaded.